Battery separators are porous diaphragms placed between the positive and negative plates of a battery so that the batteries electrolyte forms the sole internal conductive path between the positive and negative plates. It is generally considered desirable to employ separators which have the smallest pore size since this will retard the rate of active material from the battery plates penetration into and through the separator thereby causing shorting between the positive and negative plates.
Battery separators which are commonly used today in lead acid batteries are commonly composed of phenol-aldehyde impregnated cellulose sheets, sintered polyvinyl chloride or highly filled polyethylene. Separators prepared from phenol-aldehyde impregnated cellulose sheets, as described for example in U.S. Pat. Nos. 3,272,657 and 3,247,025 have been used extensively in lead acid batteries which had antimony as one component of the grid alloy. Sintered polyvinyl chloride based separators have also been used extensively in batteries having lead-antimony alloyed grids. The development of maintenance free batteries based on grid alloys which do not contain antimony has required the development of a separator having a small pore structure, good chemical resistance to oxidative attack within the battery, good flexibility and capable of being heat sealed. Separators which meet the above criteria are described in U.S. Pat. Nos. 3,351,495 and 4,024,323. Both of these patents describe battery separators which are composed of an ultra high molecular weight polyolefin, a siliceous filler and a petroleum oil and prepared by extruding the mixture of materials and then extracting most of the petroleum oil. Separators prepared in this fashion have maximum pore sizes of less than 1 micron, are typically 50-55% porous and also exhibit relatively low electrical resistance which is a desirable property. Phenol-aldehyde impregnated cellulosic webs have not been widely used with maintenance free batteries based on antimony free alloys because of their relatively poor resistance to oxidative attack in the battery, their large pore size, their high electrical resistance and the fact that they cannot be folded around the bottom of the plate and sealed on the sides.
German Offenlegunsschrift No. 2509823 described a battery separator composed of glass fiber, thermoplastic polymeric fibers and silica gell material which must be heated under specified pressure to yield the desired product. More specifically, the glass fibers in German Offenlegunsschrift 2509823 must have a diameter in the range of 0.0078"-0.16", a length in the range of 0.0039"-0.160" and have an average length of 0.024"; the thermoplastic polymeric fiber must have a diameter in the range of 0.20"-1.18" and a length in the range of 0.040"-0.080"; the silica gel material must have a particle size of less than 10 microns and the ratio of glass fiber to polymeric fiber in the product must be in the range of 1:1.5-1:2.0 and the ratio of the sum of the fibers to silica gell material must be in the range of 1:0.5-1:1. Webs prepared according to such a formula tend to have less tensile strength than those of the present invention due to among other things the absence of fibers having lengths of 0.25" or greater and such prior art webs also tend to have low tear strengths. Webs of such prior art types are typically used in conjunction with stiff glassmats and are in fact physically glued to the glassmats.
It is an object of the present invention to provide a fiberous battery separator.
It is another object of the present invention to provide a battery separator that can be prepared on a typical paper machine which has a significantly smaller maximum pore size than conventional cellulosic fiber based battery separator webs.
It is a further object of the present invention to provide a battery separator which can be prepared on a paper machine that has good flexibility so that it can be folded around the bottom of a lead battery plate without cracking.
It is yet another object of the present invention to provide a battery separator which can be prepared on a paper machine that can be sealed either by heat or ultrasonic welding.
It is another object of the present invention to provide a battery separator which can be prepared on a paper machine in which the ribs can be thermomolded onto the separator.
It is a further object of the present invention to provide a battery separator which can be prepared on a paper machine which has a tensile strength greater than 8.0 lbs./in. in the machine direction.
It is a further object of the present invention to provide an expeditious method for manufacturing the battery separator web of the present invention economically.